1. Field of the Invention
This invention relates generally to telecommunications, and, more particularly, to wireless communications.
2. Description of the Related Art
In the field of wireless telecommunications, such as cellular telephony, a system typically includes a plurality of base stations, each serving a cell, distributed within an area to be serviced by the system. Various mobile devices within the area may then access the system and, thus, other interconnected telecommunications systems, via one or more of the base stations. Typically, a mobile device maintains communications with the system as it passes through an area by communicating with one and then another base station, as the mobile device moves while maintaining an active user connection to the network. The process of moving from one base station to another is commonly referred to as a handoff and it may occur relatively often if the active mobile device is moving rapidly. The mobile device may communicate with the closest base station, the base station with the strongest signal, the base station with a capacity sufficient to accept communications, etc. A mobile device currently not involved in an active user connection selects the most suitable cell to camp on by applying standardized criteria based on measurements and ranking algorithms of surrounding cells as indicated by the serving cell. The process of selecting a cell to camp on for idle mobile devices is commonly referred to as cell re-selection.
Circumstances occasionally arise in which legal authorities may need to identify and track a target subscriber using such a mobile device. In the Global System for Mobile Communication (GSM), legal authorities use special equipment to identify and track target subscribers who use GSM mobile devices for their communications. In GSM, this equipment works by masquerading as a cell of the GSM network where the target subscriber is currently located. This triggers the mobile device to re-select to the masquerading cell and eventually tell the masquerading cell its permanent identification (e.g., by means of standardized layer 3 signaling messages). This procedure effectively exploits a security gap in the GSM standards.
GSM is in the process of being replaced by a Universal Mobile Telephone System (UMTS). The GSM procedure for identifying and tracking target subscribers is not, however, directly applicable to UMTS since the security gap has been closed in UMTS standards. Thus, target subscribers operating within the UMTS system cannot be identified or tracked using the identical procedures as in GSM.